The present invention relates to the manufacture of glass optical waveguide preforms or blanks by the doped deposited silica process, and particularly to an improvement in the known outside vapor phase oxidation (OVPO) process wherein optical waveguide blanks having low water (OH) gradients can be made.
U.S. Pat. Nos. 3,823,995 and 3,826,560 describe the manufacture of glass optical waveguides from silica and doped silica of high purity by depositing concentric layers of these materials on suitable tubular or cylindrical substrates and working the layered structures into optical waveguide filaments. In the process known as the outside or OVPO process, an optical waveguide blank is built up on a rotating, substantially cylindrical bait rod or mandrel by depositing layers of pure or doped silica glass in soot form thereon, the glass soot being vitreous (amorphous) and being produced by the flame oxidation of combustible volatile compounds of silicon, boron, germanium, phosphorus or the like.
After the soot layers have been deposited to provide a cylindrical soot preform, the central mandrel is removed leaving a central axial aperture in the preform. The preform is then dried to remove water and/or hydroxyl groups introduced into the soot structure during the deposition process while being consolidated into a clear glass tubular preform. Finally the clear tubular preform is drawn into an optical waveguide filament.
A number of different patents describing the manufacture of doped silica optical waveguides by OVPO processes have been issued. Representative patents, in addition to those noted above, include U.S. Pat. Nos. 3,806,570, 3,868,170, 3,933,454, 4,125,388 and 4,165,223.
The drying and consolidation of a silica soot preform is customarily carried out in a furnace which comprises a heated consolidation zone. The soot preform is passed slowly through the consolidation zone, causing progressive consolidation of the porous soot preform to a clear tubular glass blank as the zone is traversed. The progressive consolidation of porous silica is quite old, an early example of such process being shown in U.S. Pat. No. 2,326,059.
For optical waveguide applications, wherein the presence of residual hydroxyl groups in the finished optical waveguide is detrimental to performance, soot preforms are typically dried in the consolidation furnace contemporaneously with the consolidation process. A soot preform prepared by soot deposition as above described is quite porous, and recent practice, as shown in U.S. Pat. Nos. 4,125,388 and 4,165,223 has been to introduce a flowing stream of a drying gas or gas mixture into one end of the preform aperture, permitting this gas to exit the preform through the porous preform body as well as out the opposite end of the aperture. The flow of drying gas is continued while the preform is slowly lowered through the heated consolidation zone.
Although present drying methods are acceptable in terms of reducing the amount of residual hydroxyl species in consolidated preforms, it has been found that preforms treated by such methods exhibit a longitudinal gradient in hydroxyl ion concentration, referred to as an intra-blank water gradient, such that a typical dried consolidated preform or blank exhibits a higher hydroxyl content at one end than at the other. It is not unusual for a length of optical waveguide filament drawn from one end of such a blank to exhibit water attenuation values at a wavelength of 950 nm which are 2-7 db/km or more higher than filament drawn from the other end of the blank.
It would be desirable from the standpoint of product selection to reduce such intra-blank water gradients, so that all sections of optical waveguide drawn from a single blank would exhibit approximately equivalent water attenuation characteristics. Such a reduction would be particularly desirable if it could be accomplished without increasing the mean water attenuation value exhibited by waveguide drawn from the preform.
It is therefore the principal object of the present invention to provide an improvement in the soot preform drying process whereby consolidated preforms exhibiting lower water gradients may be obtained.
Other objects and advantages of the invention will become apparent from the following description thereof.